I'd have to admit that my lathe screen has toolpath display problems as well. Some programs show it clearly and all is as it should be, some are sort of messed up and I have to go back to my old Fanuc CNC lathe methods that had no real time toolpath display at all, and ignore it. It's sort of random, sometimes the display won't have any shading, but is ok in general, sometimes it will show odd stuff that is not meant to be there.
Dave.